In an early prior art method of manufacturing electronic circuits, small apertures were formed on printed circuit boards. The electronic elements contained metal leads which were first inserted into the small apertures on the board and then soldered to the board to complete the electronic circuit. Both the insertion step and the soldering step were performed manually. Due to the tediousness and complexity of such labor, a high failure rate was encountered and costs remained high.
Further developments in the prior art ultimately yielded methods of automatically inserting the leads of the electronic elements in the apertures of the printed circuit boards and, after the immersion of the boards into soldering tanks, automatically soldering the leads to the boards.
While the automation of the insertion and soldering steps somewhat reduced the failure rates in the manufacturing of electronic circuits, problems remained. For instance, in the automatic insertion step, it became necessary to ensure proper alignment of the elements on the board, to undergo taping of the elements to the board once alignment was assured, and to correct bends in the leads when insertion was complete. In the automatic soldering step, the necessity of masking the circuit board, checking the deterioration of the solder and controlling the temperature during soldering led to both increased complexity and increased defects. Moreover, after the soldering process was completed, it was still necessary to cut the lead junction, check for faulty junctions, repair faulty junctions, and clean the circuit board. These post-fabrication steps resulted in increased labor costs and eliminated the possibility of immediate post-fabrication continuity testing.
The latest development in the manufacture of electronic circuits, surface mounting technology, has eliminated the need for metal leads and corresponding apertures in the printed circuit boards. Instead, the electronic elements are soldered directly onto the printed circuit boards. Problems encountered in the earlier prior art due to the fragile nature of the leads have thus been eliminated. Other problems remain. For instance, strict temperature control during the soldering step is still necessary to avoid deterioration of the solder. Moreover, as in the previous prior art techniques described, the spacing between neighboring elements or soldered leads could not be reduced below approximately 2 mm due to the tendency of solder to expand. When the solder expands between neighboring junctions (whether between two lead junctions or two elements), a shorted circuit is the undesired result. In order to reduce the possibility of shorts due to the soldering step, a spacing of approximately 2 mm between junctions is required. This minimal spacing requirement prohibited further minimization of circuits which is particularly vital in complex circuit applications.
Another problem in prior art methods of fabricating electronic circuits was encountered as the number of elements attached to the board increased. The increased weight of the elements negatively affected rigidity of the electronic circuit by causing the board to bend, buckle, or, in the worst case, break completely. The need for increased rigidity was particularly important in applications where high shocks to the circuit board might occur. The only way to increase rigidity in such prior art applications was to increase the thickness of the circuit board. This, of course, required longer, more fragile leads on the electronic elements.
Furthermore, in all electronic circuits requiring a soldering step, the risk of contamination to the circuit due to moisture or chemicals seeping between the board and the electronic elements is often unacceptably high. The risk of defective electronic circuits due to contamination remains high despite high cost efforts to create contaminant-free manufacturing environments.